Epigenetics is the study of heritable changes in gene expression that occur without changes in the DNA sequence [24] . DNA methylation and histone modifications are two major epigenetic regulators in mammalian cells, which are functionally linked in transcription and may provide a mechanism for the stable propagation of gene activity from one generation of cells to the next [25] . Biochemical changes, i.e. in the form of DNA methylation and histone modifications, control the spatial, temporal
and parent-specific highly coordinated gene expression patterns. As exogenous influences can induce
epigenetic modifications, epigenetic variation among individuals may be genetically or environmentally determined [26] . Many different studies have made the-